1. Field of Invention
The invention relates to method and apparatus for metering liquid in incremental amounts which are designed precisely to determine the size of each increment, thereby accurately to measure out the liquid. Such method and apparatus is especially valuable for laboratory work, during which it is often desirable to incrementally feed small amounts, say of the magnitude of some ten microliters or so, of a liquid to a collecting vessel or to samples, for example, in order to make a titration or in order to bring materials together for reaction, it being desirable to be able to feed on the order of one thousand part volumes or increments per hour.
2. Prior Art
Apparatus for the incremental feeding of a liquid are known, but have for different reasons shown themselves to be not particularly suitable for this purpose.
One such device comprises a dosage syringe to which a manifold is fitted, this manifold having a number of exiting narrow tubes which are intended to convey equal amounts of the liquid to a corresponding number of samples. It is, however, difficult to achieve a uniform flow resistance in the different tubes, which furthermore tend to become stopped up by particles in or precipitations from the liquid conveyed, because of which the flow varies considerably in the different tubes.
Another known device comprises a vessel to which a number of elastic tubes are connected. The liquid in the vessel is actuated by a pressurized gas, for example nitrogen, in order to be driven out into the tubes, and the tubes are pressed together and opened parallelly in order to let desired equal amounts of liquid pass through the tubes. However, in such device the gas tends to dissolve in the liquid, which makes the device unsuitable for use in many applications.
A third known device comprises a number of pipettes having needle-like drop-forming means, the drop-forming velocity being controlled by an air filter at the inlet of the pipette, and the drop size being determined by the design of the needle. This device is, however, very sensitive to vibrations, and it is also difficult to adapt to different desired sample flows and liquid volumes.
Moreover, pumping devices are known which comprise an elastic hose, which is actuated by pressure means, such as the devices shown for example in U.S. Pat. Nos. 2,105,200, 2,393,898, 2,412,397 and 2,689,530. These known devices have been proposed for various kinds of pumping, but the special advantages allegedly achievable by driving such devices in accordance with the invention have not been obtained up to the present.